A reaction control subsystem maintains the orbit at approximately 217 miles (350
km). Data for each orbit is stored on board and transmitted to the ground by the
communication subsystem through TDRSS once per orbit.

The observatory instruments for primary rainfall measurements are a precipitation
radar, a multi-frequency microwave radiometer and a visible/infrared radiometer.
For observations related to precipitation, NASA added a Lightning Imaging Sensor
(LIS) and a Clouds and the Earth's Radiant Energy System (CERES). A brief
description of the five instruments follows:

The PR determines the vertical distribution of precipitation by measuring the
"radar reflectivity" of the cloud systems and the weakening of a signal as it
passes through the precipitation. A unique feature of the PR is the measurement
of rain over land, where passive microwave channels have more difficulty.

The TRMM Microwave Imager (TMI) is a multi-channel radiometer, whose signals in
combination can measure rainfall quite accurately over oceans and somewhat less
accurately over the land. The TMI and PR data, will yield the primary
precipitation data sets.

The VIRS measures radiance in five bandwidths from the visible through the
infrared spectral regions. Scientists use Infrared (IR) data to make rough
estimates of tropical precipitation. The VIRS, PR and TMI data will help improve
the techniques by which scientists use IR data from other satellites to calculate
rainfall. This is the third component of TRMM's rain package.

The LIS is an optical telescope and filter imaging system that will investigate
the distribution and variability of both atmospheric and cloud-to-ground
lightning over the Earth. These instruments will contribute to our understanding
of storm dynamics and will be correlated to levels of precipitation and the
release of latent heat.

The CERES is a visible/infrared sensor designed especially to measure energy
rising from the surface of the Earth and the atmosphere including its
constituents (e.g., clouds and aerosols). This energy, when balanced against the
energy received by the Earth from the Sun, constitutes the Earth's radiation
budget. Understanding the radiation budget, from the top of the atmosphere to the
Earth's surface, is important to understanding climate and its variability.